A 58 is a wonderful caliber for a serious bench gun: one that weighs at least 20 pounds, mounts peep site and is accompanied by a set of wind flags. Maybe even OK for shooting off cross sticks. With an adequate powder charge to get good accuracy and decent velocity to prevent a hideous drop at 100 yards, it is going to be a handful to shoot at offhand matches. Maybe fun at casual primitive shoots and effective at splitting stings, cutting cards, splitting on an ax blade, but I would find it tedious in offhand line matches. A 58 ball is about 20% heavier than a 54, and over twice as heavy as a 45.
Those Canadian brothers shoot a lot of this heavy caliber stuff and can give you advice on a good weight. I would say 10 pounds plus, and a wide flat butt.